Metalcast - ~ enters the battle with a Metal counter on it if only colorless mana was spent on it. Creatures with metal counters are colourless creatures and artifacts in addition to their other types.

Ekkremes seems to be a colour/colourless matters set so I have used this idea in my round 3 pitch, so I'm looking at adapting it so you can cast creatures as either colourless artifacts (the non-magic tech side) or coloured non artifact creatures (the emerging magic side). Still tweaking it though, the idea of using a counter is to help players keep track of what creature is what. Here are a few examples:

Reanimated Corpse
{2/B}
Creature – Skeleton
Metalcast - ~ enters the battle with a Metal counter on it if only colorless mana was spent on it. Creatures with metal counters are colourless creatures and artifacts in addition to their other types.
1/1

Retpocileh Skyknight
{2/U} {2/U}{2/U}
Creature – Drake
Flying
Metalcast - ~ enters the battle with a Metal counter on it if only colorless mana was spent on it. Creatures with metal counters are colourless creatures and artifacts in addition to their other types.
2/2

I would consider changing the Metal counter to a +1/+1 counter. You're paying twice as much for the colorless option, might as well get a bonus for jumping through that hoop. On that note, how will players get enough colorless sources in their limited decks to generate 6 for the Skynight? I would playtest it with letting you get the counter as long as you paid any colorless mana for it.

Metalcast (This enters the battlefield with a +1/+1 counter if any colorless mana was spent to cast it. It's an artifact in addition to its other types as long as it has a +1/+1 counter on it).

Nice idea. Cheers. As for colorless mana sources, it will depend on how much colorless mana is kicking around the set, but a number of people have already suggested the idea of solely using colorless as a distinction so I think there could some options.

I think the confusion between generic mana and colorless mana would be even more pronounced on colorless-hybrid cards. It looks like the two options for what the creature is in play (artifact vs. nonartifact) should line up with the two ways of casting it (2 or B), but they don't.

Say you're playing Reanimated Corpse in a red/black deck. You look at the {2/B} cost, and it seems like there are two options: cast it off of a swamp, or cast it off of two mountains. I don't expect it to be obvious to most players that the latter option, where you're casting it for 2 instead of B, still doesn't give you the metalcast bonus.

So if you play it for RG you get a bonus. B or RB wouldn't give to you. A quick beater for mono black and a chain walker for mono red.
It's kind of an anti-purity in that you get a bonus for the 2brid cost.